We measured amniotic fluid surfactant by the semiquantitative foam stability test within 24 hours before delivery of 410 infants, 64 of whom developed HMD diagnosed by standard criteria. When surfactant titers were ranked in eight categories, they predicted graded risks of HMD. On this basis we defined five "risk groups" with significantly different incidences of HMD (I = 0.5%; II = 10%; III = 25%; IV = 41%; V = 79%). Infants in Groups I and II were heavier and more mature than those in Groups III to V. However, among infants of equivalent GA or birth weight, the incidence of HMD still correlated significantly with the foam test results. Within each risk group the incidence of HMD was equal among infants delivered by vagina and by cesarean section, slightly greater among males than females, and inversely proportional to GA. In Group V the incidence of HMD was 100% among infants at less than 33 weeks' GA. We used this relationship to devise a system that improved prediction of HMD by combining the foam test results with GA.